Unsteady Forces and Flow Field on Perching Wings Close to the Ground.

During landing, birds often perform a perching maneuver that involves pitching their wing upward while decelerating to a complete stop. Such high angle of attack (AOA) maneuvers has been found to enhance the aerodynamic forces and assist in smooth landing. In this study, we want to explore how the rapid pitch-up maneuver during downward deceleration affects the evolution of flow fields and the unsteady flow fields. We consider two cases: one, where the wing only heaves downward during deceleration, and another, where the wing carries out both the heave down and pitch up motion during deceleration. In addition, we want to investigate how the wing sweep influences the perching maneuver. Results show that combined heaving and pitching resulted in higher peak forces, but the forces also decayed faster than heave only case. Our results indicate that for both the kinematics the instantaneous forces are higher in the case of the plate with a sweep. Our result also shows that the higher unsteady force on the swept plate was contributed by a stable leading-edge vortex.